Understanding the Difference Between OM4 and OM5, Part 2
In part two of this two-part blog, we describe a multi-mode fiber metric to show when to use OM4 and when to use OM5. Part one reviewed the key characteristics of the multi-mode fibers specified by the ISO/IEC 11801 standard.
Each successive generation of multi-mode fiber specified by ISO/IEC 11801 provided performance improvements. OM1 and OM2 were designed for LEDs (Light Emitting Diodes), while OM3 was the first fiber designed for use with VCSELs (Vertical Cavity Surface Emitting Lasers). OM4, also targeting VCSELs, offered further improvement, supporting 100 Gb/s Ethernet over distances of up to 150 m. It seems reasonable to assume that OM5 fiber would provide yet another performance boost. That’s not necessarily the case, however. Read on to discover why.
Effective Modal Bandwidth
Multi-mode fiber has a wider core diameter that enables it to support light propagating through the fiber over multiple paths (also described by spatial modes). The speed at which light in these modes travels varies depending upon the properties of the fiber in the optical path. DMD (Differential Mode Delay) quantifies propagation delay introduced for different optical modes excited across the diameter of the fiber.
For modern multi-mode fibers designed for use with VCSELs, the EMB (Effective Modal Bandwidth) is the metric used to define the fiber’s capacity. EMB is a frequency-length product and is defined in ISO/IEC 11801 for a specific length of fiber at a specific measurement wavelength, expressed in units of MHz×km (see Table 1). EMB is a calculated metric that is dependent on DMD and the output of the laser.
OM5 vs. OM4
Which brings us to the comparison between OM5 and OM4.
Recall that EMB is defined for a specific length of fiber at a specific measurement wavelength, and it varies with wavelength. The TIA standard for OM4 only calls out a bandwidth at 850 nm (4700 MHz×km). There is no bandwidth specification at any other wavelength. As a result, bandwidth can vary among different commercially available OM4 fibers at wavelengths outside of 850 nm.
OM5 also has a requirement of 4700 MHz×km at 850 nm. But in contrast to OM4, it extends its requirement to 953 nm (2470 MHz×km). This ensures that all OM5 fibers will meet a minimum performance level up to 953 nm.
Which one is better?
A natural assumption would be that the newer generation OM5 supports longer reach than OM4. That’s true only for specific situations. A multi-wavelength transceiver that includes longer wavelengths like 940 nm can leverage OM5’s EMB specification at longer wavelengths. If used with OM4, the reach for this specific transceiver is not determined. It could be worse than OM5, or it might even be better.
So which one is the better option, OM4 or OM5? It depends. It’s an engineering truism that there’s no perfect solution, only the best solution for the application at hand. For example, if a campus network operator needs 40 Gb/s speeds over 440 m, they should consider OM5 with a multi-wavelength transceiver. For the vast majority of shorter reach applications, multi-mode transceivers that have a single operating wavelength at 850 nm are a good choice, and there is no performance benefit of OM5 over OM4. Consider all aspects of your system, including cost and reach requirements, and only then make your choice.
For more details on this subject, see the whitepaper, “Understanding the Differences Between OM4 and OM5 Multimode Fiber.”